Digital data is often recorded on an optical disc medium using a system of uniformizing the linear velocity and thus uniformizing the recording density on the medium, as performed for a CD (Compact Disc), a DVD (Digital Versatile Disc) and the like. Data is recorded on the optical disc medium as being digitally modulated in the width of marks such that the linear recording density is uniform. Therefore, when the data is reproduced from the optical disc medium, the following inconvenience may occur. In the case where the frequency of a clock component of the reproduction signal is significantly different from the frequency of a clock signal generated by a phase synchronization loop circuit, there is an undesirable possibility that the phase synchronization is not completed, or the clock signal is pseudo-synchronized with a frequency which is different from the frequency of the clock component of the reproduction signal. In order to avoid these inconveniences, a reproduction linear velocity period of the reproduction signal is detected based on a specific pulse length or pulse interval included in the reproduction signal, and the rotation rate of the disc and the free-running frequency of the phase synchronization loop are controlled. Thus, normal phase synchronization is realized.
FIG. 22 shows a conventional frequency and phase control apparatus 180 described in Japanese Laid-Open Publication No. 2000-836602. The frequency and phase control apparatus 180 includes a waveform equalizing section 181, an analog/digital converter 182, a low frequency band noise suppression section 183, a zero-cross length detector 184, a frame counter 185, a maximum pattern length detector 186, a minimum pattern length detector 187, a cycle information determinator 188, a frequency error detector 189, a phase error detector 190, a frequency control loop filter 191, a phase control loop filter 192, digital/analog converters 193 and 194, and an oscillator 195.
The waveform equalizing section 181 emphasizes a prescribed frequency band of a reproduction signal. The analog/digital converter 182 converts the reproduction signal into multiple-bit digital data based on a reproduction clock signal. The low frequency band noise suppression section 183 suppresses low frequency band noise included in the multiple bit digital data. The zero-cross length detector 184 detects the position at which the signal having a suppressed low frequency band noise component crosses the zero level (zero-cross point), counts the number of samples between two adjacent zero-cross points (zero-cross length) based on the reproduction clock signal, and holds the counted number in a register (not shown).
The frame counter 185 counts and sets a specific period of one frame or greater. The maximum pattern length detector 186 and the minimum pattern length detector 187 respectively detect a maximum value and a minimum value of the zero-cross lengths (pattern lengths) counted by the zero-cross length detector 184 in a prescribed period (or a period corresponding to the sum of the adjacent counted zero-cross lengths). The cycle information determinator 188 compares the maximum value and the minimum value of the counted zero-cross lengths (pattern lengths), and selects an optimum value as cycle information utilizing the ratio of the maximum value and the minimum value. The frequency error detector 189 converts the difference between the cycle information and the maximum value or the difference between the cycle information and the minimum value into a frequency error amount and outputs the frequency error amount. The maximum value and the minimum value are to be detected at the time of phase synchronization. The frequency error detector 189 finds a synchronization pattern from the maximum pattern, converts the interval between two adjacent synchronization patterns into a frequency error amount, and outputs the frequency error amount.
The frequency control loop filter 191 controls the reproduction clock signal until a state where a reproduction clock signal can be considered to be synchronized with a reproduction digital signal is obtained based on the output from the frequency error detector 189. The phase error detector 190 detects phase information from the signal having a suppressed low frequency band noise component. The phase control loop filter 192 controls the reproduction clock signal such that the reproduction clock signal is synchronized with the reproduction digital signal based on the output from the phase error detector 190.
The oscillator 195 generates and oscillates the reproduction clock signal based on a sum value of the output from the frequency control loop filter 191 and the output from the phase control loop filter 192 via the digital/analog converters 193 and 194.
As described above, the frequency and phase control apparatus 180 detects the position at which the reproduction signal crosses the reference level (zero level) and thus detects a specific pulse length (synchronization pattern length) included in a reproduction signal. In order to prevent the quality of the reproduction signal from being lowered due to increase in the recording density of a recording medium (due to increase in inter-symbol interference) and also in order to improve the formatting efficiency, a new format standard has appeared by which, for example, the distance between a synchronization pattern and a maximum data pattern is shortened. This new format standard has disabled accurate detection of a synchronization pattern, which has made it difficult to stably complete the frequency synchronization.
For example, FIG. 16A shows a 14T4T synchronization pattern used for DVDs. This synchronization pattern has a long inter-symbol distance from a maximum data pattern 11T, and thus is conspicuous. Here, “T” represents a cycle of a clock signal. Reference numeral 161 represents a sampling signal. In the next generation optical disc developed for higher recording density, it is necessary to use, for example, a (1,7) RLL (Run Length Limited) modulation symbol or a specific synchronization pattern in order to improve the formatting efficiency. The (1, 7) RLL modulation symbol is usually used for HDDs (hard disc drives). FIG. 16B shows a synchronization pattern P according to an example of the present invention described below. Here, a (1, 7) RLL modulation symbol is used as a recording symbol. The synchronization pattern P is a 9T9T pattern. Reference numeral 162 represents a sampling signal. The synchronization pattern P has a shorter inter-symbol distance from a maximum data pattern of 8T8T, and thus is not conspicuous. Before the 9T9T synchronization pattern P, the minimum pattern of 2T necessarily exists. When the minimum pattern of 2T does not exceed a slicing level 163 (zero-level) as shown in, for example, FIG. 17A due to the influence of inter-symbol interference or the like, the pattern of 9T is detected as a pattern of 9T or greater. Thus, no synchronization pattern is detected from a binary signal 164. When the minimum pattern of 2T partially exceeds the slicing level 163 as shown in FIG. 17B, the pattern of 9T is detected as a pattern of 10T. Thus, no synchronization pattern is detected from a binary signal 165.
Thus, the invention described herein makes possible the advantages of providing a frequency and phase control apparatus and a maximum likelihood apparatus for accurately detecting a synchronization pattern and thus stably performing synchronization even when the quality of a reproduction signal is deteriorated.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.